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#76633  03/04/01 05:48 PM
How many lights per circuit?

Junior Member

Joined: Mar 2001
Posts: 7
MD, USA


I'm in the process of helping someone finish a basement. I was an electrician's helper for about 6 years.
The basement I am wiring has 12 recessed lights, 2 hallway lights, 2 wall lights, 3 porcelan fixtures with bulbs, a bathroom light, 1 bathroom fan and one 4 foot flourescent fixture for a total of 22. Is this too much for a 15 amp circuit? I have figured out the amperage, and it is under 12 amps, but I am wondering if there is a limit to the number of lights that can be on one circuit.
If this is too many, what is the limit? There are 2 additional 20 amp circuits that I can use. One of these 20 amp circuits will have about 10 receptacles on it. Would it be ok to put some of the lights on this circuit?



#76635  03/04/01 07:08 PM
Re: How many lights per circuit?

Member

Joined: Jan 2001
Posts: 75


Suggest you read 21052(d), 2108(a)(1), 21011(c)(3) along with most of Chapters 1, 2, 3, 4.
Glenn



#76636  03/05/01 01:17 AM
Re: How many lights per circuit?

Junior Member

Joined: Mar 2001
Posts: 7
MD, USA


Bill, Thanks for the quick response. I have reworked the lights so that I will have 14 lights on 1 circuit (15 amp) and 12 on another (20 amp changed to 15 amp). The 14 lights will all be using 65 Watt bulbs, so am I correct in calulating that 14 lights using 65W bulbs will draw 10.6 amps (65 x 14 x 1.4 / 120V)? This is roughly 71% of 15 amps, so this should be OK, correct? The new lights are all wired with 14 gauge wire. Also some of the existing lights are wired with 14 wire. I realize I can't have the 14 wire on a 20 amp circuit, so I plan to change the breaker in the panel from a 20 amp to 15 amp breaker. I assume there is no problem with this. Originally posted by Bill Addiss: Gary,
Standard procedure is to allow 180w per fixture. That would be 1.5A each or 10 on a 15amp circuit. If the actual wattage of each is known that could be used, but you have to think what is the maximum wattage bulb that can be put in those fixtures?
I would not put all of these on 1 15A circuit. [b]Note: If you connect any fixtures to the 20A circuits you must use the appropriate wire (12 awg) to the switch(s) and fixtures.
Bill[/B]



#76637  03/05/01 02:08 AM
Re: How many lights per circuit?


GaryGnu said:
..."I plan to change the breaker in the panel from a 20 amp to 15 amp breaker. I assume there is no problem with this..."
In fact, that is the absolute correct thing to do.
[This message has been edited by sparky66wv (edited 03052001).]
Virgil Residential/Commercial Inspector 5 Star Inspections Member IAEI



#76638  03/05/01 02:19 AM
Re: How many lights per circuit?

Member

Joined: Oct 2000
Posts: 3,878
NY, USA


Gary, am I correct in calulating that 14 lights using 65W bulbs will draw 10.6 amps (65 x 14 x 1.4 / 120V)? What is the 1.4 in your Formula? '66 UBB uses square brackets [b]  for example Bill



#76639  03/05/01 10:38 AM
Re: How many lights per circuit?

Junior Member

Joined: Mar 2001
Posts: 7
MD, USA


Not sure about the 1.4. This is a formula I found searching the 'net for converting Watts to Amps. Is it incorrect? (Amps = Watts x 1.4 / Volts source: http://www.sizemyups.com/iload.htm ) Originally posted by Bill Addiss: Gary,
What is the 1.4 in your Formula?
'66
UBB uses square brackets [b]  for example
Bill [This message has been edited by GaryGnu (edited 03052001).] [This message has been edited by GaryGnu (edited 03052001).] [This message has been edited by GaryGnu (edited 03052001).]



#76640  03/05/01 06:58 PM
Re: How many lights per circuit?


Hope you guys don't mind me throwing a few things into this thread don't worry, I'll be brief GaryGnu, I see that you have some Fluorescent fixtures that will be used. Be sure to figure the total line VoltAmps, rather than the Lamp wattage only, when calculating the Amperes for them [or any Fluorescent or HID Lamp / Fixture]. A common formula that was used quite a bit on the older Ballasts with the typical Magnetic Reactor [nonelectronic] was to add up the total lamp wattage, then multiply it by 1.25, then use that number for the line power [in this case  VoltAmps, instead of Watts]. An example would be an older F40T12 2 lamp ballast  80% power factor [the x1.25 reciprocal]. The total lamp wattage is 80 watts, but the total line power, in VoltAmps [VA] is 100 VA. This makes the current drawn at least 0.83 amperes for this ballast, instead of the 0.67 amps that the lamp wattage would reflect. I know this is a small fraction of current, but it does add up and increases with higher wattage lamps, so a circuit like this should be figured using VA calcualtions, rather than straight lamp wattage. The Ballast usually will state the line Amperes drawn at rated voltage. On these Reactive components, when the Voltage drops lower than it's rated level, the current [Amperes] increases. On the other hand, if the voltage increases above rated level, the current decreases. It does this to keep the KVA [or just simply VA] at a constant level. There are limits to the minimum and maximum voltage, which are normally around 10% to 20% max. Keep this in mind! The other lamps [Incandescent and Quartz Halogen] can be figured as straight true power [wattage]. In this case, there is no need to use an additional amount, like the Fluorescents did. Formula is ExI=P, or P/E=I and also P/I=E E is Voltage, I is Amperes, P is power. P can be True power, in watts  when the load [lamps] are PURE RESISTANCE [an Incandescent lamp, electric water heater, and such], or can be Apparent Power, in VoltAmps  when there is a REACTIVE load. In any case, the total power should be considered first, such as the Fluorescent Lamps: 80W x 1.25 = 100VA. 100VA/120VAC = 0.83 Amps. Your Incandescent lamps [65 watts each  total of 14] will draw apx. 910 watts total. From a 120 VAC circuit, this is around 7.5 Amps [ 910 / 120 = 7.55555]. Good luck! Hope this make sense and is of some assistance! Scott SET
Scott " 35 " Thompson Just Say NO To Green Eggs And Ham!



#76641  03/05/01 07:28 PM
Re: How many lights per circuit?


I forgot to mention a few things 1: There is an article in the NEC that requires fixtures that have interchangable lamps [in this instance  Incandescent lamps that have higher wattage ratings] to be rated at a minimum value. In your case, I think it's either 150 watts, or maybe it's higher, like Bill stated  180 watts [I know 180 VA is the general purpose receptacle rating for commercial load calcs! but it might also apply to recessed fixtures]. I don't do that much work with nonfixed lighting [like the recess cans you're installing], but the few I have done in the past, I rated each one at 150 VA [or watts] per fixture. You should design the lighting circuit to handle any future increase to the fixed outlets, which would be the result of someone changing out the 65 watt lamps for 100 watt lamps in the future. Since the largest Medium base Incandescent Lamp that is readilly available to the public is 150 watts, that would be the minimum you should rate each fixture at [the Incandescent fixtures, that is]. If there is any other manufacture's listings, follow that, but don't go below 150 watts per fixture. 2: If these lights will be on for 3 hours, or more, than the branch circuit needs to be LCL derated. This means that the maximum ampacity of the branch circuit is reduced by 80% when the load continues for 3 hours or more. Example: 15 amp circuit, maximum ampacity that can run for 3 hours or more = 12.5 amps [15x0.8]. This is an LCL derating [Long Continuous Loads]. Best if you do not max out your circuits, even if there is no LCL involved, just to keep the conductors cooler. Better to use #12 cu on the home runs [even in the outlets], but not totally needed [my own preferences ] and protect it with a 15 amp breaker. Even doing this will cover the LCL derating situation Just wanted to add these few things. Anyone that is more into the Residential angle should comment on these points beforehand [I do so little Residential work, that I am inclined to use Commercial calcs on stuff]. This lighting circuit might not require as much load calculation as I am figuring  rating the entire circuit by the minimum amperes of total connected fixtures. In my case, I would limit the number of fixtures per circuit to 10. This would be a total of 1500 VA [watts is OK], and includes an LCL derating, making the circuit's maximum current 12.5 amps. Check other's comments first, then consider my calculations. Scott SET
Scott " 35 " Thompson Just Say NO To Green Eggs And Ham!




